Fieldbuses are used in the industrial field for communication purposes, i.e. for the exchange of commands and data between several bus devices. The bus devices in a fieldbus typically involve at least one control device and at least one device to be controlled, in particular a sensor and/or actuator. A fieldbus includes a wired or wireless data transmission path of a specified type, in particular a specific copper or glass fiber line, by way of which all bus devices of the fieldbus are connected to one another in a predetermined topology (e.g. ring topology, star topology, line topology or tree topology) for the exchange of commands and data. Furthermore, a fieldbus usually includes a number of bus interfaces, wherein a bus interface is assigned in each instance to each of the bus devices. A standardised protocol which is implemented in the bus interfaces forms the basis of each fieldbus, according to which protocol the bus devices communicate with one another.
In order to enable a targeted communication between two specific bus devices or a larger subgroup of the bus devices, a unique address is usually assigned to each bus device in the case of a fieldbus. The addressing of the bus devices is also frequently used to regulate bus access. The bus devices of a fieldbus are in most instances in a master/slave configuration, wherein at least one bus device as a “master” can access the bus line and transfers data to the remaining bus devices (“slaves”) or reads in data from these remaining bus devices.
The communication of modern fieldbus standards is frequently based on the Ethernet standard. Such fieldbuses are also referred to as “Industrial Ethernet”, “Real-time Ethernet” or “Second generation fieldbuses”. Widespread second generation fieldbus standards are in particular Profinet, Ethernet/IP (EIP), EtherCAT, etc.
Fieldbuses, whose communication is not based on the Ethernet standard, are by contrast referred to as “First generation fieldbuses”. Widespread first generation fieldbus standards are in particular Profibus, CAN, CANopen, Interbus etc.
Bus devices of modern fieldbuses are furthermore frequently designed to be modular. Such bus devices comprise a number of separately addressable slots, which can optionally be occupied with plug-in card modules with varying functions.
In industrial systems, such as for instance manufacturing plants and power plants, fieldbuses can be highly complex. It may herewith be necessary or at least economically useful, for instance in the course of extending existing systems or integrating subsystems from various manufacturers, to connect several different fieldbuses to form one fieldbus system. Such a fieldbus system is frequently structured in a cascaded manner. It correspondingly includes a dominant fieldbus (first order fieldbus), from which at least one directly subordinate fieldbus (second order fieldbus) is actuated. One or a number of further fieldbuses (fieldbus of an n'th order; n=3, 4, 5, . . . ) can in turn be directly or indirectly subordinate to the or each fieldbus of the second order.
Disadvantageously the programming of such a cascaded bus system is however frequently highly complex, especially the bus devices of the subordinate fieldbuses are not visible for the fieldbus of the first order and the control device thereof. This regularly results in each fieldbus having to be programmed separately. A central controller of the bus devices of the subordinate fieldbus via the fieldbus of the first order can generally only be established with precise knowledge of all subordinate fieldbuses.
To simplify the project planning of complex fieldbus systems, “Profinet IO” so-called fieldbus access points (FAP) are provided in the fieldbus standard, by way of which further fieldbuses can be connected. Each fieldbus access node herewith spans a so-called slot cluster. Such a slot cluster contains a number of virtual slots, to which bus devices of the subordinate bus can be assigned. This assignment allows the bus devices of the subordinate bus to be presented as virtual modules of the Profinet bus. They are thus shown in the communication and address scheme of the Profinet bus as if these bus devices were physical modules of the Profinet bus. In particular, the bus devices of the subordinate fieldbus can also be addressed and responded to directly from the Profinet bus via the assigned virtual module. This concept of integrating subordinate fieldbuses is in particular described in the guideline “Fieldbus Integration in PROFINET IO”, Version 2.0 (May 2011), No.: 7.012, PROFIBUS Nutzerorganisation e.V., Karlsruhe (Germany).
The afore-described concept can nevertheless only be applied to fieldbuses which are directly subordinate to a Profinet bus as a fieldbus of the second order. By contrast, the bus devices of fieldbuses of the third order, fourth order, etc., which are only connected indirectly to the Profinet bus, cannot be presented.